new box for 2 18 treo ssx

i need help designing a box with 12 cubic ft internal and a 180 inch square port.
lenght of the port would be 17.25 inches.box dimensions would be 48 inches wide
and 261/4 inches deep and 24 inches high.it will be going in a 2003 chevy tahoe.
i would like the subs to face up and port back.i have included picture of the box
like i want except i just want a square port in the center instead of the six sided port in the pick.treo told me t would be tuned to about 38 hz.any help wil be appreciated.thanks